The present invention relates to a solder bonding method for metal or a member including metal, or more preferably, an electronic member. More particularly, the present invention relates to a solder bonding method which prevents metal from oxidizing during transportation or preservation and which enables the solder bonding of metal applying flux, which contains a small amount of halogen, and without cleaning.
When a member including metal is exposed to air during preservation or transportation, the metal easily oxidizes, an oxide layer forms over the metal surface, and the bonding properties of solder on the metal rapidly deteriorates. Therefore, in order to attain good bonding properties between the metal and solder by removing the oxide layer on the metal surface, flux (postflux) is applied on the portion to be bonded with the other material or member by solder bonding. However, the postflux usually contains halogen compounds, and when the postflux containing a large amount of halogen compounds is used in the solder bonding of a member having metal, especially an electronic member used in the electronic field, the remaining halogen compounds cause the corrosion of metal and lowers the electrical insulatability. Therefore, the halogen compounds remaining on the electronic member after application of solder are removed by cleaning treatment with organic halogen based solvents such as chlorofluoro hydrocarbon, chlorohydro hydrocarbon and fluoro hydrocarbon, and water containing surface active agents. However, the use of organic halogen-based solvents tends to be prohibited by international conventions in order to prevent the destruction of earth's environment such as the ozone layer, thereby limiting the applicability of flux containing a large amount of halogen compounds to the solder bonding of electronic material and/or an electronic member. Moreover, there are some problems in the water cleaning that waste water containing heavy metals and organic solvents after the water cleaning has to be treated in order not to contaminate the earth's environment with heavy metals and organic solvents and that drying after the water cleaning costs a considerable amount of money.
Because of the above-mentioned circumstances, no cleaning technology which does not require cleaning after the solder bonding when organic halogen based solvents are used has been developed by applying a postflux containing a small amount of halogen compounds (hereinafter referred to as "subvital postflux"). In this case, the amount of the remaining halogen on a substrate on which parts are mounted by solder bonding has to be restrained in the amount of 14 .mu.g/inch.sup.2 or less. It is necessary to reduce the amount of halogen contained in the subvital postflux.
Because of the weak strength of the subvital postflux to remove oxides, it is necessary to prevent the oxidation of the member including metal during preservation or transportation. Therefore, the following preservation methods are usually adopted: accommodating the metal member and a desiccant in a hermetically sealed manner; or replacing the inside of the preservation container with inert gas. However, these preservation methods have not always been satisfactory. For example, it is not easy to completely replace the air within the preservation container even by the method of replacing the inside of the container with inert gas such as nitrogen and argon. Even if the air is completely replaced, oxygen and moisture permeate through the packing material into the container during preservation, and the oxygen content and the humidity within the preservation container increase, causing the metal surface to be oxidized. In the method using desiccants, even if moisture is completely removed, the problem that oxidation of the metal occurs still remains. Moreover, the humidity rises due to moisture that permeates through the packing material with the passage of time or the moisture contained in the object to be preserved, causing oxidation of the metal surface.
In any case, it was difficult to prevent oxidation of the metal member by the conventional methods. As a result, there was a problem that the oxide layer of the metal member could not be removed by the solder bonding using the subvital postflux.
As described above, there was a problem that sufficient bonding properties could not be obtained by the conventional method of bonding metal by solder using a subvital postflux. The object of the present invention is to provide a method for enabling a good solder bonding of the metal and other material or member using a no-cleaning subvital postflux.